Cancer can start in any organ or tissue in the body. A primary cancer or tumor is a first, original tumor that develops in the body. A metastatic cancer or tumor occurs when cancer cells spread from a primary tumor to a new part of the body. In other words, metastatic tumors always start from cancer cells in another part of the body. For example, breast cancer can spread from its primary site (i.e., a breast) to form a new tumor in a different part of the body, such as bones. Cancer cells in a second bone tumor are the same as cells in the primary breast tumor—i.e., cells are breast cancer cells, and not bone cancer cells.
Cancer spreads from a primary tumor to other parts of the body via circulating tumor cells (CTCs). CTCs arise from primary or secondary tumors and are shed into the vasculature system from the tumors. CTCs travel in blood or lymph fluid to distant sites to form metastasis after gaining properties of extravasation, survival and proliferation in a target tissue.